[menus] All submenu in files-ng shall have margin-top: 0

Patch 1 / 3: in the code flow that shows submenu, we must position the
submenu using getComputedStyle. Just before that style recalc, set the
submenu margin-top to 0 using inline style.

The inline style overrides all CSS and all submenu's of files-app have
0 margin-top as a result. In particular, #tasks-menu, #share-menu will
no longer accidently get 8px margin-top (issue 1066727#c23).

Bug: 1066727
Change-Id: I7620d88b5026a436a94b2d1cdefed5acfa3851f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2301610Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789334}

ui/file_manager/file_manager/foreground/js/ui/multi_menu.js

@@ -205,14 +205,20 @@ cr.define('cr.ui', () => {
      * @param {cr.ui.Menu} subMenu The child (sub) menu to be positioned.
      */
     positionSubMenu_(item, subMenu) {
+      const style = subMenu.style;
+
+      if (util.isFilesNg()) {
+        style.marginTop = '0';  // crbug.com/1066727
+      }
+
       // The sub-menu needs to sit aligned to the top and side of
       // the menu-item passed in. It also needs to fit inside the viewport
       const itemRect = item.getBoundingClientRect();
       const viewportWidth = window.innerWidth;
       const viewportHeight = window.innerHeight;
       const childRect = subMenu.getBoundingClientRect();
-      const style = subMenu.style;
       const maxShift = itemRect.width / 2;
+
       // See if it fits on the right, if not position on the left
       // if there's more room on the left.
       style.left = style.right = style.top = style.bottom = 'auto';